1. Field of the Invention
The present invention relates to an exposure control apparatus of a camera in which an exposure can be controlled when a strobe (i.e., electronic flash) is used. In particular, the invention relates to a minimum diaphragm value which can be set in accordance with film sensitivity data and object distance data. More precisely, the present invention relates to an exposure control apparatus in which a program shift can be carried out in a program exposure mode when the strobe is used.
2. Description of Related Art
An exposure control apparatus is known which has a program shifting function in which, in the program exposure mode, a combination of the shutter speed and the diaphragm value can be varied without changing the exposure value. In the program shift operation, the shutter speed Tv and the diaphragm value Av are first obtained, and then, the obtained values are shifted by a predetermined amount. For example, the program shift is performed in accordance with the following equation: EQU Ev=(Av+"shift")+(Tv-"shift")
Upon taking a photograph with a strobe, the shutter speed has an upper limit defined by a strobe synchronization speed, and a lower limit defined by a camera-shake limit speed which prevents the occurrence of camera-shake. In practice, a minimum limit diaphragm value is set at a value slightly greater than the minimum diaphragm value (on an open aperture side) and a maximum limit diaphragm value is set at a value slightly less than the maximum diaphragm value (on a stop-down side), in view of the photographability of the associated camera and the photographable object distance thereof, etc. Consequently, when the strobe is used in the program exposure mode, in which a relationship between the shutter speed and the diaphragm value is programmed in advance, the ranges within which the shutter speed and diaphragm values can be set are restricted.
If the calculated exposure value is below a lower limit value which is defined by the camera-shake limit shutter speed and below the minimum limit diaphragm value, the shutter speed and the diaphragm value are set at the camera-shake limit shutter speed and the minimum limit diaphragm value, respectively. If the calculated exposure value is above an upper limit value which is defined by the strobe synchronization limit speed and above the maximum limit diaphragm value, the shutter speed and the diaphragm value are set at the strobe synchronization limit speed and the maximum limit diaphragm value, respectively. Consequently, a program shift cannot be carried out for a bright object (i.e., high luminance) or a dark object (i.e., low luminance).
Furthermore, in the conventional strobe program, the diaphragm (i.e., stop) is controlled in such a manner that, when a film having a high sensitivity is used, the diaphragm is stopped-down by steps corresponding to the increment of film sensitivity, to thereby make it possible to take a picture using the strobe at an optimum exposure value within a predetermined object distance, irrespective of the film sensitivity. Consequently, even if a film of high sensitivity is used, the object distance range in which a picture can be appropriately taken using the strobe cannot be extended.
If an object at a close distance is taken using the strobe, a large quantity of light emitted by the strobe is reflected by the object. Accordingly, stopping of the emission of strobe light based on the TTL direct metering is delayed, resulting in a tendency of over-exposure.